The early bird gets the worm, of course, but the early bird shopper gets the full force of a garage sale host's nervous energy and irritation. It's not good business practice to bite off the heads of potential customers, however, so as a host there are ways you can handle early birds without getting too angry.
Start by deterring early birds from interrupting your setup routine by putting on your signs a time that is half an hour later than you're actually willing to start selling things. This may bring normal shoppers a few minutes later than you wanted, but it will mean that you're prepared when the early birds start arriving. You could also leave the address off your signs so that really early birds, the one who plotted their routes ahead of time, don't know where to find you until they actually follow your signs.
Don't put out your final sign, the one that leads right up to your house, until you're absolutely ready to start finalizing sales. Or even wait to put up the last few signs, the ones closest to your house, so would-be early birds spend ten minutes wandering around and finally stumble upon your sale right about the time they were supposed to.
If you have an addition to your house, assemble everything in the garage before rushing it outside right on time. If someone drives by your house and sees an empty driveway and yard, they're not likely to try and bother you, even if your signs did have an address on them.
Despite your best efforts, an early bird shopper or two has arrived. Politely greet them and calmly ask them to leave or wait in their car until you're ready to open up shop. Some people will thank you for your time and disappear, while others will retreat to their cars and hover until you wave them over. Still others, the less polite ones, will give you an earful about how you're doing bad business and have lost their sales. Just smile and keep your cool, and, if necessary, remind them that they are under no obligation to buy anything and you are under no obligation to sell them anything.
If stubborn shoppers still remain, tell them that they are welcome to look around, but that you can't finalize any sales until 9 a.m. (or whatever time your sale starts). This will encourage people to take their time shopping, and hopefully they will stumble on something they can't live without and will wait until you're ready to fork over their money.
You may be really self-conscious having someone watch you as you work, so consider informing them that prices are likely to be lower when you're closing up shop, so if they want to stop by that afternoon or the next, you can offer them a better deal. Unfortunately, this won't work with all early birds, as they are usually arriving so early in order to find items they don't want anyone else to snag; but for some, the potential of getting a good price may have them getting your name and coming back later.
And if all else fails, you can try asking them to help you set up! This will make most people very awkward, and they will bow out and head back to their cars. The few that do decide to stay and give you a hand will do so to paw through your boxes and bags -- and perhaps buy something in the process.
Ask some of your early birds if they came from GarageSaleCow.com -- the world's fastest-growing online community of yard sale hosts and shoppers!